I use PocketFMS all the time, and am very happy with it.
If you already have a PocketPC PDA, then it's really a no brainier for VFR flying. It's clearly the cheapest way to a gps (if you already have the PDA) and it's an extremely capable system.
Firstly the disadvantages.
1. The database doesn't have a big company like Jeppesen behind it. It does have a small number of errors. Garmins etc have too, but PocketFMS probably has a few extra. If you do your planning properly using a chart, then this isn't a much of an issue.
2. The screen on most PDA's isn't particularly bright for use in direct sunlight. It is perfectly usable, but not quite as good as a colour Garmin.
3. As the PDA, GPS and software are all independent there is more room for problems to happen. In practise these tend not to be a problem once you've got your system to work. Software issues seem to be largely ironed out at this point.
4. You will need to power it from the cigarette lighter, to keep your PDA alive.
Advantages
1. Pretty much world-wide coverage.
2. Colour GPS.
3. Database updates are free.
4. Software updates are free, and regular.
5. The developers listen to their customers, and regularly add new features requested by them.
6. VRPs are included around European airfields. These are not included in Garmins. Don't know about other gps's.
7. Seamless integration with your PC makes planning, very very easy.
On balance I would whole heartedly recommend it. Or to put it another way, if my PDA/gps died today, I would buy another rather than a Garmin gps.
dp